Since the revision of DSM-III, high rates of co-occurring psychiatric disorders have been observed, particularly in cases of moderate and severe psychiatric illness.1 The reason lies in the design of the diagnostic system itself: DSM-IV is a descriptive, categorical system that splits psychiatric behaviors and symptoms into numerous distinct disorders, and uses few exclusionary hierarchies to eliminate multiple diagnoses. In this article, we examine the evolution of our current diagnostic system to better understand this emergence of comorbid psychiatric diagnoses. Using clinical examples, we explore strategies that can be implemented in revisions of the DSM for reducing psychiatric comorbidity, and we consider the strengths and weaknesses of each.

PSYCHIATRIC LIMITATIONS OF COMORBIDITY

Comorbidity is often used to describe the presence of more than one identified psychiatric disorder in a patient. Feinstein2 first coined the term "comorbidity" in the general medical literature, defining it as "any distinct additional clinical entity that has existed or that may occur during the clinical course of a patient who has the index disease under study" (eg, a patient with asthma and diabetes mellitus).

But in psychiatry, can we distinguish clinical entities in this way? Is the person who binges and purges and also drinks to excess suffering from 2 distinct disorders (bulimia nervosa and alcohol abuse) or a single disorder of impulse control? Similarly, when symptoms of generalized anxiety and depression co-occur, whether they indicate the presence of 2 distinct clinical entities or are 2 components of a single disorder is mostly a matter of speculation. Thus, the term "comorbidity" is misleading in psychiatry, because it implies that we have identified multiple distinct disease states.

Recognizing the limits of our understanding, DSM-IV guides clinicians to convey the maximum amount of descriptive information possible, even if a diagnosis of numerous simultaneous disorders results. According to DSM-IV, a patient who meets criteria for dysthymia, abuses alcohol, has obsessions and compulsions, and binges and purges would be assigned 4 separate Axis I disorders. Assigning multiple diagnoses allows for the communication of clinical complexity that a more general, all-encompassing diagnosis (such as "severe neurotic disorder") lacks.

ENCOURAGING COMORBID DIAGNOSES

While the DSM-IV diagnostic scheme was designed to capture clinical complexity via multiple diagnoses, there is evidence that this diagnostic complexity is not being adequately captured in medical records systems and everyday clinical practice. Zimmerman and Mattia3 have reported that clinicians routinely underdetect psychiatric comorbidity compared with research assessments using structured diagnostic interviews. One study found that 5 times as many comorbid diagnoses were made when using semistructured interviews compared with clinicians' assessments alone.4 The reasons underlying this discrepancy are most likely complex. With shrinking reimbursement and rapid patient flow, psychiatrists may lack sufficient time to undertake complete diagnostic assessments. For practical purposes, clinicians may also think more reductively when approaching patients, dismissing diagnoses that seem unrelated to the primary problem that resulted in the patient seeking care.

What are the costs of a system that encourages the concept of comorbidity? Recording 5 or 6 diagnoses on a patient's chart may obscure the intended focus of treatment, particularly for nonpsychiatric providers who request a consultation for their patient. It may confuse or alienate patients to receive diagnoses of multiple disorders. Furthermore, it may overwhelm medical records systems or administrative databases. Many health information systems only allow for coding a limited number of diagnoses, with the result that any additional diagnoses are ignored. For example, 3 patients with a primary diagnosis of major depressive disorder may have very different comorbid diagnoses (eg, obsessive-compulsive disorder, alcohol dependence, or posttraumatic stress disorder). A system that records only the major depression would imply that these 3 patients were diagnostically homogeneous, when in fact their co-occurring diagnoses suggest otherwise.

Accounting for these additional diagnoses often provides rich diagnostic information that is relevant in designing individualized treatment plans. For example, the patient with depression and an additional diagnosis of panic disorder may benefit from a short course of a benzodiazepine until an SSRI has taken therapeutic effect. Conversely, a clinician might be expected to hesitate before taking the same step in a depressed patient with a co-occurring substance abuse disorder.

At its best, our current diagnostic system has the potential to communicate large amounts of clinical information about patients with complex problems, allowing for targeted treatments and precisely defined study populations. At its worst, it can be overwhelming to clinicians and health information systems, elicit skepticism from patients and clinicians in other fields who do not understand the system, and obscure the focus of our treatments by "losing the forest for the trees."

DSM AND CO-OCCURRING DIAGNOSES

The original version of DSM was a descriptive system that incorporated many of the concepts and the structure of Emil Kraepelin's classifications of mental disorders. Compared with subsequent revisions, DSM-I and DSM-II followed a "one disease-one diagnosis" model. The clinician strove to diagnose parsimoniously, using qualifying phrases (such as "with neurotic reaction" and "with psychotic reaction") to describe complex cases. Implicit in these early DSM systems were many assumptions about the etiology and nature of mental disorders.

DSM-III, however, took a different approach of splitting diagnoses into a large number of relatively narrowly defined psychiatric disorders, providing operationalized criteria for each. For example, DSM-III split the single DSM-II category "phobic neurosis" into 5 distinct DSM-III categories. DSM-III did not encourage practitioners to assume that a wide variety of phobic reactions (such as separation anxiety disorder and simple phobia) were simply variants of the same disorder. Not surprisingly, from DSM-II to DSM-IV, the number of distinct psychiatric diagnoses nearly doubled.

Diagnostic splitting comes at the risk of spurious comorbidity (attributing a single set of symptoms toward criteria for several disorders). Symptoms such as sleep disturbance, psychomotor changes, and poor concentration in the setting of depression can also, theoretically, be counted as symptom criteria for other conditions described in DSM,increasing the likelihood of multiple diagnoses. One strategy used in DSM-III to reduce spurious comorbidity arising from diagnostic splitting was the introduction of diagnostic hierarchies. For example, the criteria for agoraphobia indicate that the diagnosis should not be given if the characteristic avoidant behavior is really due to obsessive-compulsive disorder.

However, the use of the phrase "due to" forces the clinician to determine when a symptom is attributable to one disorder versus another. As mentioned earlier, currently such decisions are based on assumptions about causality that are not empirically based. Resulting partly from research conducted by Boyd and colleagues5 in the 1970s and 1980s, many assumptions about the relationship between mood and anxiety symptoms came into question. In subsequent revisions of DSM, starting with DSM-III-R, these concerns led to the removal of many exclusionary hierarchies.

The ultimate result of this combination of widespread diagnostic splitting, with few exclusionary hierarchies, is that patients qualify for multiple diagnoses. In a study involving 500 patients presenting for intake in a general psychiatric clinic, Zimmerman and Mattia,6 using semistructured clinical interviews, noted that more than a third of the patients qualified as having 3 or more Axis I disorders.

STRATEGIES TO REDUCE COMORBID DIAGNOSES

A prominent example of excessive comorbidity in DSM-IV that has resulted in dissatisfaction among clinicians is in the area of personality disorders.7,8

DSM describes 10 personality disorders, with no hierarchical system provided to reduce comorbidity. According to this system, when an individual meets criteria for more than 1 disorder, all diagnoses should be listed. Widiger and colleagues8 have pointed out that certain psychiatric inpatients meet criteria for 3 to 5, and in some cases 7, personality disorders. Using pooled data for 1116 inpatients and outpatients, Stuart and colleagues9 found that those patients who met criteria for any personality disorder actually met criteria for an average of 2.7 personality disorders.

However, clinicians still tend to diagnose personality disorders reductively. In a study in which psychiatrists were given case histories of 46 patients who met criteria for 4 personality disorders, two thirds diagnosed only 1 personality disorder, a quarter diagnosed 2, and none diagnosed all 4.10

Dimensional approach

An alternative approach to the DSM-IV categorical method for diagnosing personality disorders is to adopt a dimensional model that depicts relative psychopathology as points on a spectrum. The 5-factor model (FFM) of personality is one externally validated dimensional system that has been proposed as an alternative to the DSM-IV categorical system.11,12 Rather than applying distinct criteria to distinguish "case" from "noncase," the FFM is used to rate patients on 5 dimensions: neuroticism, extraversion, openness to experience, agreeableness, and conscientiousness. By using the dimensional model, the FFM is able to characterize patients in complex ways without applying multiple labels that imply distinct disorders.